There are many to choose from in the marketplace. Our company makes the ETH-1000 Ethernet / RS-485 gateway that supports Modbus/TCP (client and server), BACnet/IP (client and server) and BACnet MS/TP (client and server), among many others. So you can be guaranteed to move data regardless of physical layer or client/server behavior needs. It's USB-configurable and all drivers are included for free.
> I am looking for a Modbus TCP/IP to BACnet Converter. Which is the best option (performance wise)?
Do you need a converter, or could you maintain the use of the wiring/topology to have both Ethernet/IP and RS-485 at the same time? (E.G. IP-485 allows you to use the existing twisted pair for both Multidrop Ethernet and RS-485 (BACnet)at the same time? Performance is Standard RS-485 Specs, with Ethernet at LRE distances and bandwidths at average 1-4 Mbps with peak burst at 20Mbps.